1. Field of the Invention
The present invention relates to wireless communications, and more particularly to a method and an apparatus for identifying an interference source in a wireless network.
2. Description of Related Art
Wireless communication networks which utilize spread spectrum digital modulation techniques, such as the Code Division Multiple Access (CDMA) system specified in the IS-95 Standard of the U.S. Telecommunication Industry Association (TIA), form a plurality of channels from the same radio frequency (RF) bandwidth using pseudonoise (PN) sequences. More specifically, mobile subscriber terminals (xe2x80x9cmobilesxe2x80x9d) in a CDMA system are each assigned a unique PN sequence and combine a communication signal, such as voice, data, and control signals, with their assigned PN sequence to distinguish their transmissions from those of other mobiles. The intended receiver removes the PN sequence to xe2x80x9cdespreadxe2x80x9d the received signal and recover the transmitted communication signal.
By using the same RF bandwidth for a number of CDMA signals, each transmitting mobile will cause a degree of interference for other mobiles, thereby affecting the base stations ability to adequately serve a large number of mobiles anid, thus, limiting a base station""s service capacity. To increase service capacity while ensuring adequate call quality, CDMA systems typically utilize reverse link (mobile transmitting to base station) power control techniques to adaptively set the transmit power of each mobile being served to the minimum level needed to maintain adequate performance. More specifically, a base station compares reverse link call quality (typically represented as the ratio of energy per bit, Eb, to interference, No) for each mobile being served with a target quality value, and generates either a power up-adjust or down adjust command at predetermined intervals (e.g., every 1.25 milliseconds), depending on whether reverse link call quality is above or below a target level.
Although a base station can thereby control the transmit power for each mobile being served to increase capacity, RF signals from other sources in the network service area may create uncontrollable interference. As a result of such uncontrollable interference sources, an affected base station may be forced to generate power up-adjust commands to maintain adequate call quality for served mobiles. If a substantial number of mobiles increase their transmit power, the base station will experience a sharp increase in interference, thereby, forcing the base station to generate even more power up-adjust commands. As a result, certain mobiles, particularly those at cell/sector boundaries, will not be able to transmit at the power level needed to overcome the resulting rise in interference, thereby causing the base station to drop these mobiles (i.e., temporarily decreasing the size of the base station""s service area).
An interfering RF signal source that causes the above-described problem may be a nearby mobile that is not being served by (xe2x80x9cnot in handoff withxe2x80x9d) the affected base station. This condition is commonly referred to as xe2x80x9creverse link other cell interference,xe2x80x9d and would most commonly occur when the affected base station""s forward link (base station transmitting to mobile) coverage is significantly less than its reverse link coverage, and if the interfering mobile is stationary. These differences between forward and reverse link coverage may be caused by various RF network conditions, such as frequency-specific interference (i.e., interference present on the base station""s transmission frequency but not present on the mobile""s transmission frequency) and frequency-specific fading (i.e., fading that occurs on the base station""s transmission frequency but does not occur on the mobile""s transmission frequency), by which the mobile does not xe2x80x9chearxe2x80x9d the pilot signal transmitted by the affected base station, or at least does not pick up a strong pilot signal from the affected base station. For a stationary mobile, large differences between forward and reverse link coverage may persist for long periods.
The present invention is a method and an apparatus for identifying reverse link other cell interference in a wireless communications network and for determining whether a handoff to an affected base station should occur. In one embodiment, the present invention detects an increased interference condition in a CDMA-based wireless network, determines whether the cause of increased interference is a nearby mobile which is not in handoff with the affected base station, and selectively triggers a handoff to the affected base station when the interfering mobile can be adequately served by the affected base station.
In one exemplary implementation, a base station of a CDMA wireless network continuously monitors reverse link interference levels, e.g., based on receive signal strength indicator (RSSI) measurements, to detect an increased interference condition. Upon detecting an increased interference condition, the affected base station initiates an xe2x80x9cother cell interference identifyingxe2x80x9d sequence to determine whether the interfering signal source is a proximate mobile being served by a neighboring base station of the CDMA wireless network. More specifically, the affected base station obtains a list of PN sequences for mobiles being served by neighboring base stations, and instructs at least one receiver unit, called a CDMA locate radio (CLR), to sequentially despread received signals using each of the PN sequences in the list of PN sequences. When the CLR detects a signal transmitted by a non-served mobile, the base station analyzes reverse link call quality for the non-served mobile to determine whether the non-served mobile is likely the source of interference.
Having determined that a nearby CDMA mobile is the source of interference, the affected base station may take one of a number of possible actions. First, the affected base station requests, via the mobile telephone switching office (MTSO) and the base station serving the interfering mobile (xe2x80x9cthe serving base stationxe2x80x9d), that the interfering mobile indicate whether the affected base station is a candidate for serving the interfering mobile. If the affected base station is a candidate for serving the interfering mobile, the affected base station can be promoted to the interfering mobile""s xe2x80x9cactive setxe2x80x9d (i.e., the mobile can select the affected base station to provide service), and an inability of the affected base station to control the transmit power of the interfering mobile may negatively affect the service quality provided by the affected base station, the interfering mobile is forced to handoff to the affected base station. After handoff, the affected base station can control the interfering mobile""s transmit power level. If the affected base station is not a suitable candidate for serving the interfering mobile or if the affected base station cannot otherwise be promoted to the interfering mobile""s active set (e.g., if the interfering mobile""s active set is full), the affected base station may request that the interfering mobile be moved to a different frequency carrier or dropped if the interference problem is severe enough to preserve overall network performance. Furthermore, if the interfering mobile is not moved to a different frequency carrier or dropped, the affected base station may merely record the interference condition to aid future network analysis, design, and/or modifications.